1. Field of the Invention
The present invention relates to Li2Oxe2x80x94Al2O3xe2x80x94SiO2 crystallized glass and Li2Oxe2x80x94Al2O3xe2x80x94SiO2 crystallizable glass therefor.
2. Description of the Related Art
Conventionally, Li2Oxe2x80x94Al2O3xe2x80x94SiO2 crystallized glass has been used for front glass panels in kerosene heaters, wood stoves and the like, substrates for high-technology products, such as color filters, image sensors and the like, setters for baking electronic devices, shelf boards for microwave ovens, top plates for electromagnetic cooking devices, window glass for fire doors, and the like. For example, Japanese Examined Patent Publication(JP-B) No. S39-21,049, Japanese Examined Patent Publication (JP-B) No. S40-20,182, and Japanese Unexamined Patent Publication (JP-A) No. H01-308,845 disclose Li2Oxe2x80x94Al2O3xe2x80x94SiO2 crystallized glass having a xcex2-quartz solid solution represented by the formula Li2Oxe2x80x94Al2O3.nSiO2, in which nxe2x89xa72, or a xcex2-spodumene solid solution represented by the formula Li2Oxe2x80x94Al2O3.nSiO2, in which nxe2x89xa74, both of which are produced in the crystallized glass as a main crystal.
The Li2Oxe2x80x94Al2O3xe2x80x94SiO2 crystallized glass mentioned above has high mechanical strength and superior thermal characteristics, such as a low coefficient of thermal expansion. In addition, since the crystal produced in the Li2xe2x80x94Oxe2x80x94Al2O3xe2x80x94SiO2 crystallized glass is alterable by changing heating conditions in a crystallization process, transparent crystallized glass (a xcex2-quartz solid solution is produced) and white and opaque crystallized glass (a xcex2-spodumene solid solution is produced) can be produced from the same composition of raw glass material. Consequently, in accordance with the application, Li2Oxe2x80x94Al2O3xe2x80x94SiO2 crystallized glass suitable for the application can be selectively produced.
When this kind of crystallized glass is produced, heating to a high temperature above 1,400xc2x0 C. for melting raw glass materials is required. Accordingly, as a fining agent added to one batch for glass production, arsenic oxide (As2O3) is used which is capable of evolving a great deal of gas for fining during melting at a high temperature. In melting in a batch production system, As2O3 contained in raw glass materials is oxidized at 400 to 500xc2x0 C. so as to form As2O5, and the As2O5 is again reduced at 1,200 to 1,800xc2x0 C. to form As2O3, whereby oxygen gas is evolved. The oxygen gas thus evolved diffuses into bubbles in the glass, and the bubbles enlarge, facilitating rising thereof, whereby the bubbles are removed. Since As2O3 has the fining effect described above, As2O3 has been widely used as a fining agent. In particular, As2O3 is a very effective fining agent for Li2Oxe2x80x94Al2O3xe2x80x94SiO2 crystallized glass that must be subjected to melting at a high temperature.
In addition to the fining effect, As2O3 has an effect of promoting crystallization. Accordingly, it is understood that AS2O3 is an essential component in order to obtain desired characteristics of the crystallized glass described above.
However, As2O3 is high toxic, and hence, environmental pollution may occur during a manufacturing process for glass, disposal of waste glass, and the like. Accordingly, it is desirable to reduce the, amount of As2O3 used. However, when the amount of As2O3 is simply decreased, the effects on fining and crystallization are degraded, and as a result, clarity and glass characteristics, which are equivalent to those obtained if the amount of As2O3 were not decreased, cannot be obtained.
Accordingly, it is an object of the present invention to provide Li2Oxe2x80x94Al2O3xe2x80x94SiO2 crystallized glass having clarity and glass characteristics equivalent to or superior to those of a conventional crystallized glass, even when the amount of As2O3 used is decreased, and to provide Li2Oxe2x80x94Al2O3xe2x80x94SiO2 crystallizable glass as a raw glass material therefor.
According to one aspect of the present invention, there is provided a Li2Oxe2x80x94Al2O3xe2x80x94SiO2 crystallized glass which contains, by weight, 0.05 to 4% Sb2O3, in which the content of xcex2-OH is 0.3 to 4/mm.
According to another aspect of the present invention, there is provided a Li2Oxe2x80x94Al2O3xe2x80x94SiO2 crystallizable glass which contains, by weight, 0.05 to 4% Sb2O3, in which the content of xcex2-OH is 0.15 to 2/mm.
Description will first be made in detail as regards a Li2Oxe2x80x94Al2O3xe2x80x94SiO2 crystallizable glass and a Li2Oxe2x80x94Al2O3xe2x80x94SiO2 crystallized glass according to the present invention.
The Li2Oxe2x80x94Al2O3xe2x80x94SiO2 crystallized glass of t he present invention has a xcex2-quartz solid solution or a xcex2-spodumene solid solution produced as a main crystal therein. By the production of the crystal Mentioned above, the Li2Oxe2x80x94Al2O3xe2x80x94SiO2 crystallized glass is obtained, which has a low coefficient of thermal expansion of approximately xe2x88x9210 to 30xc3x9710xe2x88x927/xc2x0 C. (30 to 750xc2x0 C.) and high mechanical strength. When a metastable xcex2-quartz solid solution is produced as a main crystal, transparent crystallized glass is obtained, and when a metastable xcex2-quartz solid solution is inverted to a stable xcex2-spodumene solid solution by heating at a high temperature, white and opaque crystallized glass is obtained.
Instead of As2O3, antimony oxide (Sb2O3) is used in the crystallized glass of the present invention. Sb2O3 is oxidized to Sb2O5 at approximately 400xc2x0 C. under melting conditions, and in a high temperature range of 1000xc2x0 C. or more, a great deal of oxygen gas is evolved by a reaction caused by a change in valence of the antimony (Sb) ion. The gas evolved diffuses into bubbles remaining in the glass and increases the diameters of the bubbles, so that the bubbles rise and escape, resulting in the fining of the glass. In addition, it was understood through the research by the inventors of the present invention that Sb2O3 functions to promote crystallization in the glass just as As2O3 does.
However, the effects of Sb2O3 on fining and promotion of crystallinity are smaller than those of As2O3. When the content of Sb2O3 is increased, the effects described above are enhanced; however, since Sb2O3 facilitates coloration of glass due to impurities (in the present invention, coloration caused by impurity Fe2O3 in the presence of TiO2) more strongly than As2O3, Sb2O3 cannot be added to the glass in an amount sufficient to obtain the effects of Sb2O3 on fining and promotion of crystallinity. Through the intensive research by the present inventors of the present invention, it was discovered that clarity and crystallinity of glass can be improved when the content of water in the glass is increased. In this connection, even though the promotion of crystallinity of glass can be achieved by increasing the content of MgO, ZnO, Na2O, K2O, and the like, MgO and ZnO promote coloration due to impurities, and coefficient of thermal expansion is significantly increased when Na2O or K2O is added. Accordingly, the components mentioned above cannot be added to glass in an amount sufficient to promote crystallinity. In contrast, water can improve clarity and crystallinity of glass without promoting coloration due to impurities and increasing coefficient of thermal expansion.
In the present invention, the content of water in glass is represented by the content of xcex2-OH.
The content of xcex2-OH in Li2Oxe2x80x94Al2O3xe2x80x94SiO2 crystallized glass of the present invention is calculated by the following equation based on the data obtained in an infrared absorption spectrum; the content of xcex2-OH (/mm)={log(T3850/T3500)}/t, in which T3850 is the transmittance in the vicinity of 3,850 cmxe2x88x921, T3500 is the minimum transmittance in the vicinity of 3,500 cmxe2x88x921, and t is the thickness of the crystallized glass used for spectrum measurement (mm, actual measured value 3 mm).
In addition, the content of xcex2-OH in Li2Oxe2x80x94Al2O3xe2x80x94SiO2 crystallizable glass of the present invention for producing the crystallized glass described above is calculated by the following equation based on the data obtained in a infrared absorption spectrum; the content of xcex2-OH (/mm)={log(T3850/T3570)}/t, in which T3850 is the transmittance in the vicinity of 3,850 cmxe2x88x921, T3570 is the minimum transmittance in the vicinity of 3,570 cmxe2x88x921, and t is the thickness of the crystallizable glass used for spectrum measurement (mm, actual measured value 3 mm).
In the case in which As2O3 is replaced with Sb2O3, when the amount of water, in which a satisfactory crystallinity can be obtained, is represented by xcex2-OH in accordance with the equations described above, the content in the crystallized glass is 0.3 to 4/mm, and preferably, 0.3 to 2/mm, and the content in the crystallizable glass is 0.15 to 2/mm, and preferably 0.15 to 1/mm. When the content of xcex2-OH in the crystallized glass is less than 0.3/mm, and the content of xcex2-OH in the crystallizable glass is less than 0.15/mm, the clarity of the glass is deteriorated. In addition, since the crystallinity of the crystallized glass is low, white turbidity occurs in the transparent crystallized glass, and sufficient whiteness in the white crystallized glass cannot be obtained. In contrast, when the content of xcex2-OH in the crystallized glass is more than 4/mm, and the content of xcex2-OH in the crystallizable glass is more than 2/mm, since the crystallinity of the crystallized glass is significantly high, a state of a metastable xcex2-quartz solid solution cannot be maintained, and as a result, transparent crystallized glass is difficult to obtain. In addition, in the case of white crystallized glass, it is not preferable since the crystallization thereof occurs in excess, so that the brilliance at the surface of the crystallized glass is reduced. The reason for the difference in the contents of xcex2-OH in the crystallized glass and crystallizable glass is that, when xcex2-OHs of crystallized glass and crystallizable glass are measured in which the compositions and water contents are equal to each other, the xcex2-OH of the crystallized glass is approximately 1.3 to 2 times that of the crystallizable glass due to the difference in infrared absorption characteristics therebetween.
The preferable composition of the crystallized glass of the present invention is, by weight, 50 to 80% SiO2, 12 to 30% Al2O3, 1 to 6% Li2O, 0 to 5% MgO, 0 to 5% ZnO, 0 to 8% BaO, 0 to 5% Na2O, 0 to 5% K2O, 0 to 8% TiO2, 0 to 7% ZrO2, 0 to 7% P2O5, and 0.05 to 4% Sb2O3.
The reasons for the limitations on the composition in the present invention will be described.
The content of SiO2 is a constituent for forming the crystal and the glass network former, and is 50 to 80%, preferably, 60 to 75%, and more preferably 60 to 71%. When the content of SiO2 is less than 50%, the coefficient of thermal expansion is increased considerably. On the other hand, when the content thereof is more than 80%, the glass is difficult to melt.
The content of Al2O3 is a constituent for forming the crystal and the glass network former, and is 12 to 30%, preferably 17 to 27%, and more preferably, 17 to 24%. When the content of Al2O3 is less than 12%, the chemical resistance is deteriorated, and the glass is likely to devitrify. On the other hand, when the content thereof is more than 30%, the glass is difficult to melt due to a significant increase in viscosity thereof.
Li2O is a component for forming the crystal and has a function of decreasing the viscosity thereof in addition to a significant influence on the crystallinity of the glass. The content of Li2O is 1 to 6%, preferably 3 to 5%, and more preferably, 3.2 to 4.8%. When the content of Li2O is less than 1%, the crystallinity of the glass is low, and the coefficient of thermal expansion is increased considerably. In addition, when the glass is transparent crystallized glass, white turbidity is likely to occur. When the glass is white crystallized glass, the whiteness thereof tends to fall. On the other hand, when the content of Li2O is more than 6%, the glass is likely to devitrify due to significantly high crystallinity. Therefore, a metastable xcex2-quartz solid solution is difficult to obtain so that white turbidity occurs and transparent crystallized glass is difficult obtain.
The content of MgO and the content of ZnO are respectively 0 to 5%, preferably 0 to 3%, and more preferably, 0 to 2.5%. When the contents of MgO and ZnO are each more than 5%, the crystallinity is high, and coloration due to impurities is intense due to an increase in the amount of crystal produced.
The total content of MgO and ZnO is 0 to 7%, and preferably, 0 to 5%. When the total content of MgO and ZnO is more than 7%, coloration due to impurities tends to be intense.
The content of BaO is 0 to 8%, preferably 0.3 to 5%, and more preferably, 0.5 to 4%. When the content of BaO is more than 8%, a sufficient amount of crystal is difficult to obtain since production of the crystal is inhibited, and hence, the coefficient of thermal expansion is significantly increased. In addition, in the case of forming transparent crystallized glass, white turbidity in the crystal is likely to occur. BaO is not an essential component; however, BaO is preferably contained by the following reasons described below. That is, even though the effects of BaO on fining and promotion of crystallization are smaller than those of As2O3, it was discovered through the intensive research by the present inventors that BaO functions to enhance the fining effect of Sb2O3, and when BaO is used with Sb2O3, oxygen gas evolved from Sb2O3 effectively clarify the glass, and hence, an effect thereof on fining equivalent to that of As2O3 can be obtained. The reasons BaO has the effect mentioned above are believed to be that (1) the time for fining is shortened by facilitating dissolving of undissolved raw materials, and (2) bubbles quickly increase in size by lowering the viscosity of a molten solution of the glass and by increasing the diffusion velocity of the gas to promote fining. Furthermore, it was also discovered that BaO has a function of improving crystallinity due to a decrease in viscosity in addition to the function of enhancing the effect of Sb2O3 on fining. Accordingly, it is preferable that BaO be contained as an essential component.
The content of Na2O and the content of K2, are respectively 0 to 5%, preferably 0 to 3%, and more preferably, 0 to 2%. When the contents of Na2O and K2O are each more than 5%, a sufficient amount of the crystal cannot be obtained due to a decrease in crystallinity, and the coefficient of thermal expansion is increased significantly. Furthermore, when transparent crystallized glass is produced, white turbidity in the crystal is likely to occur.
The total content of Na2O and K2O is 0 to 7%, and preferably, 0 to 4%. When the total content of Na2O and K2O is more than 7%, the coefficient of thermal expansion is likely to increase, and white turbidity in the crystal is readily to occur when transparent crystallized glass is produced.
The content of Tio2 as a nucleation agent is 0 to 8%, preferably 0 to 4%, and more preferably, 0 to 3%. When the content of TiO2 is more than 8%, the coloration due to impurities is intense.
The content of ZrO2 as a nucleation agent is 0 to 7%, preferably 1 to 4%, and more preferably, 1 to 3%. When the content of ZrO2 is more than 7%, the glass is significantly devitrified in addition to it being difficult to melt the glass.
The content of P2O5 is a component for improving the crystallinity of glass, and is 0 to 7%, preferably 0 to 4%, and more preferably, 0 to 3%. When the content of P2O5 is more than 7%, the coefficient of thermal expansion is significantly increased, and white turbidity in the crystal is likely to occur when transparent crystallized glass is produced.
The content of Sb2O3 is 0.05 to 4%, preferably 0.05 to 2%, and more preferably, 0.1 to 1.5%. When the content of Sb2O3is less than 0.05%, the effects on fining and promotion of crystallization cannot be obtained, and when the content is more than 4%, the coloration due to impurities is considerable.
In the crystallized glass of the present invention, BaO and Sb2O3, which have significant influence on clarity and crystallinity, are preferably adjusted so as to have the ratio of BaO/Sb2O3 from 0.2 to 15 on a percent by weight basis, and more preferably, from 0.3 to 13. When the ratio of BaO/Sb2O3 is less than 0.2, satisfactory effects on fining and promotion of crystallization are difficult to obtain. On the other hand, when the ratio of BaO/Sb2O3is more than 13, a sufficient amount of crystal cannot be obtained due to a decrease in crystallinity, and as a result, desired characteristics of the glass are difficult to obtain.
Similarly to the above, the water and Sb2O3, which have significant influence on crystallinity, are preferably adjusted so as to have the ratio of xcex2-OH (/mm)/Sb2O3 (percent by weight) from 0.05 to 10, and more preferably, 0.10 to 8. When the ratio is less than 0.05, the crystallinity is decreased, white turbidity is likely to occur in transparent crystallized glass, and the whiteness of white crystallized glass is difficult to obtain. On the other hand, when the ratio is more than 10, a state of a metastable xcex2-quartz solid solution is difficult to maintain due to significantly high crystallinity, and as a result, transparent crystallized glass is difficult to obtain. In addition, devitrification in the glass occurs, and as a result, the brilliance at the surface of the crystallized glass may be reduced.
Furthermore, various components other than those described above may be added to the crystallized glass of the present invention.
For example, in order to enhance the effects on fining and promotion of crystallization, SnO2 may be added in an amount of up to 2%, preferably up to 0.8%, and more preferably, up to 0.6%. SnO2 acts as a fining agent by evolving oxygen at a higher temperature than Sb2O3. In addition, since SnO2 forms ZrO2xe2x80x94TiO2xe2x80x94SnO2 crystal nuclei together with TiO2 and ZrO2, SnO2 also acts as a nucleation agent. However, SnO2 is more likely to cause coloration due to impurities than Sb2O3 and As2O3, it is not preferable that SnO2 be added in an amount exceeding the content mentioned above. In addition, chlorine in an amount of up to 2%, and preferably, up to 1%, may be added to glass. However, it is not preferable since the chemical resistance of the glass is deteriorated when the amount of the chlorine exceeds the content mentioned above. Fining agent other than those mentioned above may be added to glass; however, the contents thereof must be limited to ranges in which the fining agents do not adversely affect the characteristics of glass. In this connection, in order to complement the effects on fining and promotion of crystallinity, As2O3 may be added in an amount of up to approximately 0.8%. However, the amount of As2O3 added should be as small as possible, and it is recommendable not to use As2O3, if possible.
Furthermore, as a coloring agent, for example, Fe2O3 may be added in an amount of up to 0.5%, preferably up to 0.1%, and more preferably, up to 0.08%.
The Li2Oxe2x80x94Al2O3xe2x80x94SiO2 crystallized glass of the present invention having the composition described above can be produced by a manufacturing method described below.
Raw material having the composition described below on a percent by weight basis is prepared: 50 to 80% SiO2, 12 to 30%/o Al2O3, 1 to 6% Li2O, 0 to 5% MgO, 0 to 5% ZnO, 0 to 8% BaO, 0 to 5% Na2O, 0 to 5% K2O, 0 to 8% TiO2, 0 to 7% ZrO2, 0 to 7% P2O5, and 0.05 to 4% Sb2O3. In addition, when necessary, SnO2, chlorine, Fe2O3, and the like may be added.
The glass material is melted at 1,550 to 1,750xc2x0 C. for 4 to 20 hours, and the molten glass material is molded, whereby Li2Oxe2x80x94Al2O3xe2x80x94SiO2 crystallizable glass is obtained. In order to adjust the content of xcex2-OH from 0.15 to 2/mm in the crystallizable glass, for example, a raw material containing a large amount of water, such as aluminum hydroxide, may be selected, water content in. combustion gases used for melting glass may be in, creased, or water vapor may be bubbled in molten glass.
The molded material composed of the crystallizable glass is then held at 700 to 800xc2x0 C. for 1 to 4 hours so as to form nuclei. Subsequently, in order to obtain transparent crystallized glass, the molded material is heated at 800 to 950xc2x0 C. for 0.5 to 3 hours so as to produce a xcex2-quartz solid solution. In order to obtain white opaque crystallized glass, the molded material is heated at 1,050 to 1,250xc2x0 C. for 0.5 to 2 hours so as to produce a xcex2-spodumene solid solution. The crystallized glass thus obtained has the content of xcex2-OH of 0.3 to 4/mm.
After the crystallized glass thus obtained is processed by, for example, cutting, polishing, bending, painting thereon, and the like, the crystallized glass is applied to various applications.